1. Field of the Invention
The present invention relates to laser systems including an oscillator and an amplifier which exhibit thermally induced birefringence effects in the gain media. More particularly, the present invention provides a laser oscillator/amplifier system in which compensation for thermally induced birefringence in the gain medium of the oscillator is accomplished by offsetting thermally induced birefringence in the amplifier.
2. Description of Related Art
Linearly polarized laser beams are desirable for many laser related operations, such as harmonic generation, Q-switching, and external modulation of the beam. Further, for many applications it is desirable to have a uniform power density in a desired polarization.
The generation of a linearly polarized laser beam with a uniform power density with laser media that exhibit thermally induced stress birefringence under high thermal loading conditions has proved quite difficult. Therefore, in systems utilizing gain media like YAG, GSAG, GSGG, or YSGG, lasers are forced to emit linearly polarized beams by the introduction of a polarizer in the resonant cavity between the output coupler and the gain medium. The polarizer causes a significant decrease in output power and a degradation in the beam shape.
In the prior art, laser amplifiers have been designed to compensate for thermally induced birefringence by passing the beam to be amplified through the gain media with a first polarization, and then rotating the polarization by ninety degrees and passing the rotated beam back through the birefringent gain media. The depolarization effects of the first pass through the material are compensated by equal and opposite depolarization effects that take place during the second pass. Alternatively, rather than providing for two passes through a single gain medium, two matched gain media can be placed along the optical path with a ninety degree rotator in between. In this system, birefringence in the first gain medium is compensated by birefringence in the second gain medium. See Heritier et al., "Thermal Effects in High Power Q-Switched Lasers", SPIE OE-LASE Conference, Jan. 1988.
Likewise, a laser oscillator has been implemented, using two matching gain media within a oscillator cavity having a ninety degrees rotator in between. See Scott et al., "Birefringence Compensation and TEM00 Mode Enhancement in a Nd:YAG Laser", APPLIED PHYSICS
LETTERS, Vol. 18, No. 1, Jan. 1, 1971, pages 3-4.See also, Koechner, SOLID STATE LASER ENGINEERING, Second Edition, Springer-Verlag, 1988, pages 361-367.
In systems requiring high power, it is desirable to amplify the output of the laser oscillator. These prior art systems would require birefringence compensation in both the oscillator leg and the amplifier leg in order to ensure a uniform power distribution and high power density in the desired polarization. Such systems result in exceedingly complex laser apparatus.
It is desirable therefore to implement an oscillator/amplifier system with birefringence compensation efficiently and at relatively low cost.